The present invention relates to calibration of measurement networks, enabling de-embedding of subsequent microwave circuit measurements. More specifically, the invention provides a method for correcting errors in the error factors of an error model where the errors are caused by inaccurate knowledge of network parameters of calibration standards used as references in the calibration process. Such imperfect assumptions may be caused, for example, by reactance in the measuring circuit due to variable positioning of circuit elements, such as probes, couplings, and the like, as described in Strid et al., U.S. Pat. No. 4,858,160 which is incorporated herein by reference.
Ideally, a microwave circuit's network parameters would be directly measurable. However, at microwave frequencies, the network parameters of the microwave circuit, that is, the device under test (DUT), are hidden within measurements of the "measurement network" which not only includes the DUT, but also the necessary connections, reflectometers, and probes interposed between the DUT and the network analyzer. The DUT is said to be "embedded" within the measurement network.
The characteristics of the measurement network, whether used in a one-port, two-port, or N-port mode, are assumed to be repeatable, and thus may be accounted for in an error model having various error factors describing various parameters of the measurement network. Through use of the error model, network properties of the DUT may be determined from measurements of the measurement network including the DUT. This process is referred to as "de-embedding" the DUT.
The measurement network's parameters, and thus the error factors, may be determined from measurements of the measurement network for various "calibration standards," that is, devices having known network parameters. The number of independent calibrating measurements is at least equal to the number of error factors in the error model. Additional measurements may be averaged to minimize random errors in measurements. However, the accuracy of the determined error factors is limited to the accuracy to which the network parameters of the calibration standards are known.
What is needed is a method of detecting and correcting inaccuracies in error factors and calibration standard network parameters. This method preferably provides additional information on the calibration standards.